Historically, research that examines cognitive deficits in neurological patient populations has remained separate from research that uses mathematical models to examine "normal cognition. However, these two areas have begun to merge (e.g., Farah, 1990; Maddox, et.al., 1995). Model-based analyses are often superior to traditional specific cognitive deficit, and (c) determine the magnitude of these deficits. Maddox, el. al. (1995) utilized modeling techniques to examine PD patients' ability (a) to integrate information from two aspects of a stimulus (termed dimensional integration, DI), and (b) to attend selectively to one aspect of a stimulus while ignoring the other (termed selective attention; SA). We found that PD patients showed no DI deficits, but a large proportion of PD patients did show SA deficits. This proposal outlines two extensions of this research. Project 1 will examine the differential effects of stimulus separability and integrality on PD patients' SA performance. Normal subjects' SA performance is strongly influenced by stimulus separability and integrality; SA is easy with separable dimension, but is hard with integral dimensions (Maddox, 1992). Maddox et al. (1995) utilized integral dimensions and found SA deficits in PD patients. In Project 1, subjects will participate in two experiments; one that uses integral dimensions, and the other that uses separable dimensions. Model-based analyses will be applied to isolate and quantify the effects of stimulus integrality on PD patients' SA performance. Project 2 will examine the differential effects of an internal vs. external criterion on PD patients' DI performance. Maddox et al. (1995) found that PD patient were not impaired in their DI ability when no internal referent was required. Some have argued that PD patients had more impaired on tasks that place greater demands on the internal control of attention (e.g., Brown & Marsden, 1988). Subjects will participate in two experiments like those used in Project 1, however one will require an external criterion, and the other will not model-based analyses will again be used to isolate and quantify the effects of an internal vs. external criterion on PD patients' DI performance.